1. Field of the Invention
The present invention relates to a technology of removing dust adhering to a surface of an image pickup device such as a digital camera and an optical component incorporated in the image pickup device, and more particularly, to a dust removing device and a dust removing method for removing dust through application of vibration.
2. Description of the Related Art
In an image pickup device such as a digital camera for picking up an image by converting an image signal into an electric signal, a photographing light flux is received by an image pickup element such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS). Then, a photoelectric conversion signal output from the image pickup element is converted into image data, and the image data is recorded on a recording medium such as a memory card. In such an image pickup device, an optical low-pass filter and an infrared cut filter are disposed in front (on a subject side) of the image pickup element.
In the image pickup device of this type, when dust adheres to a surface of a cover glass of the image pickup element or to surfaces of those filters, the dust comes out as a black spot in the picked-up image. In particular, in a single-lens reflex digital camera which includes an interchangeable lens, dust may enter the digital camera main body through the opening of the lens mount when the lens is being changed, and may adhere to the surfaces of the cover glass of the image pickup element or of the filters.
In view of the above, there has been proposed a digital camera which includes a dust removing device for removing dust adhering to the surface by using a vibration of a piezoelectric device (see, for example, Japanese Patent Application Laid-Open Nos. 2003-348403 and 2008-228074).
In the dust removing device included in the digital camera disclosed in each of Japanese Patent Application Laid-Open Nos. 2003-348403 and 2008-228074, a voltage is applied to a piezoelectric device fixedly bonded to a vibrating plate so as to drive the piezoelectric device, to thereby generate elastic vibration (hereinafter, defined as flexural vibration) for causing a displacement of the vibrating plate in an optical axis direction, that is, in a thickness direction of the vibrating plate. The dust removing device removes dust adhering to the surface of the vibrating plate through application of the flexural vibration thus generated.
In the above-mentioned configuration, according to Japanese Patent Application Laid-Open No. 2003-348403, the vibrating plate is pressed, in the circumferential part thereof, by a member called pressing member, such as an annular-shaped member or multiple members. Due to a biased force applied by the pressing member, the dust removing device is held and fixed to the image pickup element or the digital camera main body.
Similarly, according to Japanese Patent Application Laid-Open No. 2008-228074, the vibrating plate is applied with a pressure by a member called holding member, such as a single component formed of a material such as metal having spring characteristics (elasticity). Due to a biased force applied by the holding member, the dust removing device is held and fixed to the image pickup element or the digital camera main body.
Further, the piezoelectric device is in the form of a plate having a ring shape or a rectangular shape, and includes a piezoelectric material and a pair of electrodes opposed to each other. The pair of electrodes include a first electrode and a second electrode which are disposed on plate surfaces of the piezoelectric material. The first electrode is also called lower electrode, and the second electrode is also called upper electrode. Here, a stretching strain is generated in the piezoelectric material by an electric field applied between the electrodes, which generates an elastic vibration (hereinafter, defined as length vibration) for causing a displacement of the piezoelectric device in a direction perpendicular to the optical axis of the vibrating plate, that is, in a direction (hereinafter, defined as length direction) perpendicular to the thickness direction of the piezoelectric device. Due to the length vibration of the piezoelectric device, a stress is generated between the piezoelectric device and the vibrating plate fixedly bonded to the piezoelectric device, to thereby generate a flexural vibration in the vibrating plate.
In the vibrating plate, a voltage to be applied to the piezoelectric device is controlled in frequency or phase, so that the flexural vibration of the vibrating plate may generate a standing wave of multiple orders, which is called vibration modes, having multiple nodes and antinodes, or a carrier wave which has nodes and antinodes and moves in a length direction of the vibration plate with respect to time. For example, in the dust removing device included in the digital camera disclosed in Japanese Patent Application Laid-Open No. 2008-228074, a pair of the piezoelectric devices are applied with voltages which are reversed in phase by 180 degrees, so that two vibration modes, namely, an 18th order vibration mode and a 19th order vibration mode, may be generated, and the two vibration modes are selectively used effectively, to thereby remove dust adhering to a surface of the vibrating plate.
Here, the dust removing device of Japanese Patent Application Laid-Open Nos. 2003-348403 and 2008-228074 is driven at a frequency in proximity to the resonance frequency of the vibrating plate, and hence a larger flexural vibration may be generated in the vibrating plate even when a smaller voltage is applied to the piezoelectric device.
Further, the magnitude of the length vibration of the piezoelectric device is closely related to a magnitude of the piezoelectric displacement resulting from the piezoelectric transverse effect of the piezoelectric ceramics.
Meanwhile, the piezoelectric device which is currently used in various devices uses a piezoelectric material containing a large amount of lead, such as lead zirconate titanate (PZT: PbZr1-xTixO3) containing lead. However, it has been pointed out that such a piezoelectric material made of PZT containing a large amount of lead can be detrimental to ecosystems because the lead component in the piezoelectric material seeps into soil when the piezoelectric material is once discarded to be exposed to acid rain, for example. In view of this, in recent years, with consideration given to the environment and to comply with laws restricting the use of lead in various products, a piezoelectric material (lead-free piezoelectric material) containing no lead or a minimum amount of lead and product development therefor are under study and consideration. However, a lead-free material having various properties equivalent to those of PZT has not been realized, and there are still only a few examples of commercialized devices using a lead-free piezoelectric material which is equivalent in quality to PZT.
As described above, in the dust removing device included in the digital camera disclosed in Japanese Patent Application Laid-Open Nos. 2003-348403 and 2008-228074, a length vibration is generated in the piezoelectric device, to thereby generate a flexural vibration in the vibrating plate. The dust removing device removes dust adhering to the surface of the vibrating plate through application of the flexural vibration thus generated.
However, the vibration to be generated in the piezoelectric device is a length vibration, and hence the dust removing device cannot be directly fixed to the image pickup element or to the digital camera main body. Accordingly, in order to fixedly hold the dust removing device, a pressing member or a holding member needs to be provided for generating a biased force. However, the vibrating plate is subjected to the flexural vibration and the piezoelectric device itself is subjected to the length vibration, the dust removing device cannot be held without inevitably hindering the vibration of the vibrating plate, no matter where the dust removing device is held by the pressing member or the holding member. Accordingly, according to the conventional configuration of the dust removing device, there has been a problem that the dust removal performance is degraded when the dust removing device is held and fixed to an image pickup element or a digital camera main body.
Further, the conventional configuration of the dust removing device requires a pressing member and a holding member for fixing the dust removing device to an image pickup element or a digital camera main body, but the pressing member and the holding member are not essential to the dust removing device in the first place.
Further, a magnitude of the length vibration of the piezoelectric device is closely related to a magnitude of the piezoelectric displacement resulting from the piezoelectric transverse effect of the piezoelectric ceramics. The piezoelectric effects of the piezoelectric ceramics include, other than the piezoelectric transverse effect, a piezoelectric longitudinal effect, and a piezoelectric thickness-shear effect. Here, the piezoelectric transverse effect refers to a strain to be generated in the piezoelectric ceramics when an electric field is applied in the same direction as the polarization axis direction, in a direction perpendicular to the polarization axis direction and the electric field application direction. The piezoelectric longitudinal effect refers to a strain to be generated in the piezoelectric ceramics when an electric field is applied in the same direction as the polarization axis direction, in the same direction. The piezoelectric thickness-shear effect refers to shearing strain to be generated in the piezoelectric ceramics when an electric field is applied in a direction perpendicular to the polarization axis direction.
The relation of the magnitudes of the piezoelectric displacements resulting from the piezoelectric effects are defined as follows: piezoelectric thickness−shear effect>piezoelectric longitudinal effect>piezoelectric transverse effect. For example, in the case of lead zirconate titanate (PZT: PbZr1-xTixO3) and barium titanate (BTO: BaTiO3), which are representative examples of piezoelectric ceramics, a magnitude of the piezoelectric displacement resulting from the piezoelectric thickness-shear effect is more than twice as large as the piezoelectric displacement resulting from the piezoelectric transverse effect. In other words, in the conventional dust removing device using the length vibration of the piezoelectric device, there has been a problem that the displacement capability inherent in the piezoelectric device cannot be used effectively.
Further, in the dust removing device included in the digital camera of Japanese Patent Application Laid-Open No. 2008-228074, the piezoelectric device is in a rectangular solid shape. However, a wavefront (here, wavefront is defined as a continued surface obtained by connecting points in the same phase of the wave at a certain point of time) formed on the vibrating plate due to the flexural vibration generated in the vibrating plate is parallel to a certain side (hereinafter, referred to as longitudinal direction) of the piezoelectric device in a rectangular solid shape, which is parallel to the length direction of the piezoelectric device. However, the length vibration to be generated in the piezoelectric device occurs omnidirectionally in the length direction of the piezoelectric device, and hence the same wavefront with an equal amplitude may be difficult to form on the vibrating plate. Accordingly, according to the conventional dust removing device, there has been a problem that the dust removal performance greatly varies depending on the area on the vibrating plate.